Will wrote:
Is there any way to have the Garmin 430/530 put its current display accuracy
on the primary display as an ongoing statistic, based on the number of
satellites in view? How, in general, do the Garmin units notify you of
situations where GPS accuracy has been compromised to a level that makes it
unsafe to use the Garmin for a GPS approach?

Why would you want that information? In single-pilot operations,
especially, looking at those data constitutes information overload.
That is what RAIM is all about, to keep it simple. RAIM is much more
robust for the final approach segment than for terminal mode. You
simply aren't going to have issues with an IFR-certified GPS (properly
installed) that you will have with a hand-held.

I got an interesting lesson in GPS recently while traveling with a handheld
GPS as the passenger in a plane. The GPS showed us landing about two miles
east of the airport. I figured out only later that the position of the
antenna was such that many satellites were blocked, so the accuracy of the
GPS signal was greatly diminished.

That large of an error was probably due to the substantial altitude
change of the airliner while your GPS was staggering along in 2-D mode.

The particular software I was using
didn't display its current accuracy on the primary display. Based on that
event, I realize I cannot just trust a GPS display without first
understanding the current accuracy of the signal.

As others have told you, the portable does not have RAIM. It is a VFR
device. It was not designed to be robust through a cabin window of an
airliner. Some owners, who are savvy on this still, install an external
antenna on their aircraft for their hand-held GPS. It will never have
the problems you experienced with an external antenna.

What would be really nice is if the primary display would show vertical and
horizontal accuracy as two separate numbers, based on some high confidence
interval (99.99+%). Knowing that the current display reading is accurate
to 10 ft vertical and 15 ft horizontal, for example, might make you a lot
more comfortable in following a GPS approach than a display where the 99.99%
confidence interval is 2000 ft vertical/horizontal (i.e., GPS reliability is
completely compromised by virtue of blocked satellites, bad GPS antenna,
etc).


Again, RAIM and proper IFR installation procedures mitigate your
concerns to the point of being irrelevant.

There is different, higher level of accuracy, integrity, and continuity
than "plain vanilla" TSO-C129 IFR GPS. That is an IFR-approved RNP
platform, which is a quandum leap in RNAV integrity. RNP platforms have
enough information to make you happy in your quest. But, the displays
and software are presently heavy iron stuff, and huge overkill for most
IFR operations today.